Descent, Touchdown and Repositioning of a hopping planetary lander on Enceladus

This report deals with the design of a complete guidance, navigation and control system for a planetary lander on Enceladus, for both descent and repositioning. A basic lander model with a main engine, attitude thrusters and a landing gear is presented, for which the equations of motion in different reference frames are derived, including the effects of third body perturbations and non-homogeneous gravity fields. The guidance system incorporates a gravity-turn guidance logic for a flat and a spherical moon model for the initial descent phase, a quadratic guidance logic for both pinpoint landing and repositioning, a velocity nullifying logic for the terminal guidance phase and a ballistic guidance logic for a more fuel-efficient repositioning. The control system consists of a linear quaternion controller combined with a pulse-width-pulse-frequency modulator. The navigation system is build around an Inertial Measurement Unit with an extended Kalman filter to fuse the attitude and position measurements. The on-board software incorporates a basic hazard avoidance system, which uses a simulated LIDAR scan of the target area to generate a hazard map. The retargeting is based on a reachability and fuel consumption analysis. The spacecraft is capable of fully autonomous landings. This guidance, navigation and control system is combined in a simulation software written in C++ to test the system performance in presence of various error sources. All program elements have been tested with results from literature. The Monte Carlo simulations of the full GNC system include variances of the lander's state vector and of the GNC configuration parameters. The success rate is 90% for the descent phase, and 98% for the repositioning phase. The mean landing precision is in the order of 1m, with velocity and pointing errors of 0.2 m/s and 1.4°, respectively. Stricter requirements on the lander's mass distribution, and the implementation of a feedback velocity nullifying guidance scheme can increase the mission success rate even further. The thesis work shows, that a landing mission to Enceladus is possible with current technology. The Enceladus Lander Simulator is comprehensive software package for descent and repositioning simulations, and its modularity allows extensions in the future.Astrodynamics and Space MissionsEarth Observation and Space SystemsAerospace Engineerin